There have been many different types and kinds of mass production techniques for assembling electronic devices, such as integrated circuit devices, to a printed circuit board. One such technique is a heated bar reflow soldering system. In this system, an electronic device or component to be installed is 1) retrieved from a lead die machine by a pick and place unit; 2) transported to a location in close proximity to a device placement site; 3) visualized and aligned so the leads of the device can be accurately placed on the pads of the printed circuit board; 4) placed on the pads of the printed circuit board; and 5) secured to the printed circuit board pads by applying heat to solder pre-deposited on the pads.
Such a hot bar reflow soldering system typically employs a robotic system including a robotic arm for moving selectively the electronic component relative to the printed circuit board. A soldering head mounted on the end of the robotic arm may include a pick and place unit for holding and carrying the electronic component for placement on a soldering site, such as the pads of a printed circuit board. The head also includes a soldering arrangement including a reflow soldering hot bar unit, for soldering the leads of the component to the pads of a printed circuit board.
An important aspect in a reflow soldering system is the accurate placement and soldering of the leads of a device to the pad of the circuit board to establish proper solder connections for electrical continuity. In this regard, if the closely spaced leads of an electronic component are not each properly aligned relative to the corresponding pads on the printed circuit board, improper solder connections can result. Thus, costly and unwanted rejects of the assembled circuit board, can occur.
While such prior known systems have been generally satisfactory for standard pitch electronic devices, new assembly problems are being encountered with the increasing use of fine pitch electronic devices. In this regard, there has been increased potential for misalignment between the leads of a fine pitch device and the pads of a corresponding substrate site, such as a printed circuit board. More particularly, potential misalignment problems can develop from three sources: (1) handling of the device prior to placement on the printed circuit board; (2) non-coplanarity between the device leads and the substrate site during placement; and (3) movement induced by the reflow process itself. Therefore it would be highly desirable for any robotic hot bar soldering system utilized in fine pitch surface mount technology to greatly reduce, if not eliminate entirely, such misalignment between the leads of a fine pitch integrated circuit device and the pads of a corresponding printed circuit board.
Integrated reflow hot bar soldering heads are well known in the prior art. However, prior art systems utilized for the placement and assembly of fine pitch devices to printed circuit board substrates have proven less than totally satisfactory in that they have not successfully reduced or substantially eliminated non-coplanarity problems between the leads of the device and the substrate surface.
One attempted solution at solving the aforementioned non-coplanarity problem has been the direct pressure method. In this regard, in order to solve the non-coplanarity problem, the soldering elements of the hot bar soldering head are brought into direct contact with the leads of the electronic device to apply not only heat to the pads of the printed circuit board, but also to apply direct force to the leads of the device for maintaining lead alignment with the pads of the printed circuit board. While this attempted solution has proven satisfactory with standard pitch devices, the coupling of the hot bar tool to a robotic control source has induced other problems with reference to fine pitch devices.
For example, conventional large scale fine pitch devices are generally rectangular in shape with groups of fine pitch leads extending from all four sides of the body of the device. For soldering purposes, this type of fine pitch device configuration requires a hot bar head with a set of hot bars configured in a generally rectangular configuration for engaging simultaneously all the leads of the electronic device. In this regard, an operator of the robotic control unit is able to visually align the leads of the electronic device by adjusting their orientation relative to the pads of the printed circuit board. Such adjustments are in coplanar X,Y directions and rotatively in a .theta.(z) direction perpendicular to the X,Y planes.
However, coplanar adjustments for maintaining a parallel relationship between the electronic device and the substrate surface of the printed circuit board are difficult, if not entirely impossible to be made because of the difficulty in visualizing the parallel relationship between the leads and the pads of the printed circuit board. Thus, if any coplanarity problem exists, the pads of the printed circuit board do not tend to be engaged simultaneously with all of the leads of the electronic device. As a result of noncoplanarity between the pads and the leads, various forces are directed against the reflow solder head and the robotic arm supporting the head. The effects produced by such forces depends on the position of the line of action of the impact force and the resulting translational forces induced as the head continues to downward path of travel. Moreover particularly, the translational forces can cause either the robotic unit or the printed circuit board to move. The movement of either one can result in misalignments between leads and pads.
Moreover, once the leads of the electronic device are engaged with the pads, further misalignment problems can also develop as heat is applied to the solder pre-deposited on the printed circuit board pads. In this regard, depending on the thermal mass of the leads, pads and solder, the solder on the pads may liquify non-uniformly and unevenly. Thus, for example, if the solder liquifies on one side of the board as opposed to the other, the soldering head will sink downwardly into the liquified solder but will also remain stationary against the solid solder. Thus, as the one side sinks into the solder during the solder reflow process, non-coplanarity will again induce rotative forces against the head, causing the robotic unit and head to rotate, resulting in, for example, lead to pad misalignment, unsoldered connections, or improper solder connections.
Therefore it would be highly desirable to have a new and improved robotic arm tool apparatus and method of using to provide a highly precise and accurate mode of operation. Such an apparatus and method should reduce, if not eliminate entirely, lead to pad misalignments caused by non-coplanarity problems between the leads of an electronics device and the pads of a corresponding printed circuit board.